In everyday and military English, take down is a phrasal verb that commonly means to overpower, defeat, or kill someone, especially an opponent in a fight or conflict. So when someone talks about taking down the enemy, they mean defeating or killing that enemy in confrontation, not performing some unrelated physical task.
The distractors all misread the phrase. Taking pictures off a wall applies a literal, domestic sense of take down that has nothing to do with enemies in conflict. Making friends with the enemy is essentially the reverse of what the idiom conveys, since take down implies hostility and victory over the other side, not reconciliation. Ignoring the enemy suggests inaction, while the idiom specifically implies decisive, forceful action against the enemy.
Reading the phrase in its combat sense, the correct meaning is to kill the enemy, which makes option 2 correct.